1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Brief Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a manufacturing process using a lithographic apparatus, the pattern has to be imaged on the substrate very accurately. The current lithographic projection apparatuses are commonly used to manufacture devices with typical dimensions in the micron or sub micron range. Hence, the pattern has to be imaged on the substrate with a corresponding accuracy.
For precise positioning and handling, in most known lithographic apparatuses, the substrate is supported, and (optionally) moved, relative to other parts in the lithographic apparatus by a substrate table. The table has a chuck system which is provided with a chuck. The chuck has a support surface on which the supported object, e.g. the substrate, can be clamped, in order to support the object. Furthermore, the known lithographic apparatus can be provided with a mask table which supports a mask. The mask table may also have a chuck system, which includes a chuck on which the mask can be mounted.
For example, from U.S. Pat. No. 6,353,271 a lithographic apparatus is known, which comprises a stage for precise positioning of a chuck in three orthogonal linear axes and in three orthogonal rotation axes. A substrate can be clamped on the chuck using an electrostatic force. The stage comprises a frame which supports the chuck. In this prior art document, the frame is implemented as a monolithic mirror block. The chuck is rigidly mounted on the monolithic mirror block. The monolithic mirror block is provided with mirrors that are part of an interferometer system that is used to determine the position of the chuck relative to other parts of the lithographic apparatus.
However, a drawback of the chuck system known from this prior art document is that the chuck may deform, for example due to heating during the lithographic projection process. The deformation of the chuck is transferred onto the frame or mirror block. Hence, the mirror block is deformed as well. This, inter alia, affects the accuracy of the interferometric measurements performed using the mirrors on the mirror block, which in turn affects the precision of positioning the substrate relative to the projection beam.